Various processes for the preparation of a printed wiring board have already been developed, among which a process comprising the use of a photocurable and photosensitive resin as an etching resist and a process comprising the use of said resin as a plating resist have recently been employed predominantly.
The former process comprises forming a layer of a photocurable resin composition on a circuit board by some means, adhering a negative-type circuit pattern mask closely to the layer, irradiating actinic rays on the whole to cure desired portions of the layer, removing uncured portions of the layer by development and then etching the resulting board to form a predetermined circuit pattern.
The latter process comprises irradiating a photocurable resin layer formed on a circuit board, with actinic rays through a positive-type pattern mask, removing desired portions of the resin layer by development, plating the desired portions with, e.g., etching-resistant solder, stripping off undesired portions of the resin layer and then etching the resulting board to obtain a predetermined circuit pattern.
The "circuit pattern mask" used in the above processes corresponds to a photographic negative and comprises a light-barrier layer for forming a suitable pattern on a transparent substrate. Although glass may be used as the substrate or base, a plastic film such as polyester film is predominantly used at present. The material of the light-barrier layer is generally chromium, silver or other metal.
The photocurable resin composition used in the above processes generally comprises (1) a binder polymer, (2) a photopolymerizable monomer or oligomer (as a crosslinking agent), (3) a photopolymerization initiator and (4) other components such as a stabilizer, colorant and flame retardant.
The binder polymer serves to shape the resin composition into a film and may mainly be an acrylic polymer which is prepared by the copolymerization of acrylic acid, an acrylate ester, methacrylic acid, a methacrylate ester, styrene etc. and has a molecular weight of several tens of thousands.
The photopolymerizable monomer or oligomer also contains an acryloyl or methacryloyl group. The principle of the above circuit pattern forming process resides in that such a resin composition is used to form a layer and only desired areas of the layer are insolubilized by light exposure.
Although U.S. Pat. No. 3,954,587 discloses a process comprising the use of a specific photocurable composition containing a maleinized oil as a so-called photoresist such as an etching resist, it has been ascertained by the follow-up made by the inventors of the present invention that the resulting coating before irradiation with light is tacky at ordinary temperatures as examined according to the tack test, because the composition used in said Patent comprises a maleinized oil and an ethylenically unsaturated compound having at least two unsaturated bonds in the molecule.
In the conventional production of a printed wiring board, a circuit pattern mask is adhered in a vacuum closely to a photosensitive coating formed on a substrate, and the whole is then exposed to UV. If the photosensitive resin coating is tacky at ordinary temperatures, the coating will partially remain adhered to the mask when the mask is attempted to be peeled off the coating after the exposure. Although this adhesion can be prevented by using a cooling apparatus or bath after the exposure step, this means will increase the equipment cost, necessitate one additional step and make the operation troublesome unfavorably. Meanwhile, the application of a release agent or the like to the circuit pattern mask will exert adverse effects on the formation of a fine pattern. Under these circumstances, the inventors of the present invention have reported in Japanese Patent Application Laid-0pen Gazette No. 138371/1990 which corresponds to U.S. Pat. No. 5,057,400 that a photosensitive resin coating made from a modified resin prepared by adding an .alpha.,.beta.-unsaturated dicarboxylic acid anhydride to a conjugated diene polymer or copolymer to form an adduct having a softening point of 70.degree. to 200.degree. C. (as determined by the ring and ball softening point method according to JIS K 2531-60) and reacting the adduct with an .alpha.,.beta.-unsaturated mono- or di-carboxylic acid ester having an alcoholic hydroxy group to open a part or the entire acid anhydride rings and introduce conjugated double bonds into the adduct, is a little tacky at ordinary temperatures as examined according to the tack test.
In the practical production of a printed wiring board, the extent of adhesion of a photosensitive coating remaining adhered to a circuit pattern mask after the step of peeling the mask from the coating varies depending upon various conditions such as temperature, pressure and time. In any case, a higher temperature, a higher pressure or a longer time will cause the coating to remain adhered to the mask more easily after the removal of the mask. For example, because the practical production of a printed wiring board is conducted continuously, the temperature of the atmosphere itself is liable to rise by the heat dissipation of a light source. Further, the vacuum adhesion of the mask to the photosensitive coating applies a pressure to a substrate for the board. These phenomena are causative of more liability of the coating to remain adhered to the mask.
Therefore, the inventors of the present invention tested the photosensitive coating of Japanese Patent Application Laid-Open Gazette No. 138371/1990 which corresponds to U.S. Pat. No. 5,057,400 for its adhesion to a pattern mask after the step of peeling the mask under approximately the same conditions as those for the practical production with the result that the coating of said application was found to be not always satisfactory.
In the above test, a distinct peel noise was not heard when the pattern mask was peeled off the coating after light exposure, which means that the coating was not completely tack-free. When the photosensitive resin coating is not completely tack-free, during the repeated use of the pattern mask, which is expensive, in the industrial production of circuit boards, the resin will cumulatively adhere to the peeled mask, which makes further use of the mask impossible or gives a deficient or shortcircuit resist pattern at the worst.
Further, the photosensitive coating formed on a substrate for a wiring board must have a measure of hardness. This is because the steps of producing a printed wiring board are carried out one after another on a conveyor line, that is, a substrate having a photosensitive resin coating formed thereon is transferred to a light exposure step through the conveyor line, during which the coating is liable to be damaged in a case where it is soft. The photosensitive resin coating made from the previously mentioned modified resin has insufficient hardness.